In a manufacturing method of a seamless metal pipe using a mandrel mill, first, a heated round billet is pierced by a piercing mill, and thus, a hollow shell is manufactured. A mandrel bar is inserted into the manufactured hollow shell. The hollow shell into which the mandrel bar is inserted is elongated by a mandrel mill. The elongated hollow shell is heated as needed and is reduction-rolled by a sizing mill or a stretch reducing mill. According to the above-described processes, a seamless metal pipe is manufactured.
In the manufacturing method of a seamless metal pipe, seamless metal pipes having various steel grades and sizes (outer diameter and thickness) are manufactured. Accordingly, improvement of production efficiency is required.
Patent Document 1 suggests an art which increases production efficiency by increasing an elongation ratio of a seamless metal pipe in a mandrel mill. In the mandrel mill disclosed in Patent Document 1, roll diameters of first and second stands are set to be larger than a predetermined value. Accordingly, the elongation ratio of the seamless metal pipe can be increased.